Summary:
* cmake 2.8.12 is really outdated, 3.0 is minimum in plasma & kf5
* cmake_minimum_required should be at begin of toplevel CMakeLists.txt
* bump qt version to 5.7, matching the min Qt version of kf5 5.42
* with ecm being part of kf5 since early versions, share ${KF5_MIN_VERSION}
* use QT_MIN_VERSION & KF5_MIN_VERSION also for separate find_package calls
* deduplicate all KF5 components searched for in unconditionally included
subdirs
Test Plan: Still configures and builds with all options OFF & ON
Reviewers: #kde_connect, nicolasfella
Reviewed By: #kde_connect, nicolasfella
Subscribers: nicolasfella
Differential Revision: https://phabricator.kde.org/D11418
Centralizing the plugins' config will ensure that all the plugins store it
the same way (ie: not in random files scattered around, like until now).
The base KCM class, together with the already existing base plugin class,
will give easy access to all the plugins to this centralized config. Also,
now the settings are not shared across devices (that is: every device can
have different config for a same plugin).
Note: This commit requires KCMUtils 5.9
REVIEW: 122927
